The present invention relates to navigation systems and in particular to geographic data sets for use in navigation systems.
Computer-based navigation systems for use on land have become available in a variety of forms and provide for a variety of useful features. These features may include route calculation functions, maneuver generation functions, map display functions, vehicle positioning functions, destination resolution capabilities, and so on. When performing a map display function with a navigation system, the sizes, shapes, and relative positions of geographic features in a portion of a geographic region are portrayed on a display associated with the navigation system. These geographic features may include roads, intersections, and points-of-interest, as well as other features, such as lakes, railroad tracks, buildings, airports, stadiums, parks, mountain ranges, docks, ferries, tunnels, bridges, and so on. To provide this map display function, a geographic data set used with the navigation application program includes the appropriate information which allows the navigation system to display the size, shape, position, and character of these various geographic features on a display device associated with the navigation system. Zooming and panning of the displayed areas may also be supported.
When the navigation system provides the end user with a graphical visual display of a portion of the geographic region, it may provide an image that represents an overhead view (i.e., a two dimensional view or bird's eye view) similar to a conventional map. With an overhead view, roads or other geographic features that cross over or under each other should be portrayed in a manner that provides the end user with meaningful information. For example, it may be helpful to an end user to know that the road upon which he or she is driving is going to cross over an expressway, or cross under a railroad trestle, and so on. This kind of information provides visual cues which may be useful to assist the driver in relating his or her physical location to the image portrayed on the map display associated with the navigation system. In order to represent these overpass and underpass situations, the detailed geographic data set used by the navigation system may include relative elevational data ("z-levels") or altitude data associated with the data which represents the geographic features stored in the data set. Relative elevational data provides an ordering of all geographic features that cross at the same latitude and longitude coordinates. Altitude data may include the actual geographical altitude (e.g., feet above sea level) of a represented geographic feature.
Using the elevational data or the altitude data, the navigation system can determine which of the geographic features is topmost and portray this feature in a way that conveys to the end user that the topmost geographic feature passes over the other features. This determination may impose a computational burden on the navigation system thereby slowing down the speed at which the image is rendered. The determination of which geographic feature to show as being topmost may be further complicated when the navigation application supports zooming in and out. When an image is zoomed out, less detail may be included in the image thereby resulting in some topmost features, such as an overpass by a secondary roadway, being omitted from the zoomed out view. When this happens, it may be necessary to recalculate which feature is topmost. In addition, an image may include a feature that passes over another feature and then passes under a third feature. This further complicates the calculation of which feature is topmost since a feature may be topmost in one location but not topmost in another location. These considerations further complicate the process of displaying the image of the portion of the geographic region in a meaningful manner and impose a burden on the memory and processing resources of the navigation system.
Accordingly, there is a need in navigation systems to provide a method and system for representing geographic features which cross over or under each other in such a way as to minimize the impact on performance of the navigation system.